In the design of modem transportation vehicles, structural vibration and interior noise have become important problems that need to be addressed. For example, in helicopter systems control of sound transmission into enclosed spaces is an important issue. Various studies have shown that the predominant frequency components associated with the noise transmission lie in the frequency range of 50 Hz to 5500 Hz. There are various approaches that may be used to control sound field inside a helicopter cabin.
Among the different approaches, one is based on controlling the radiation (transmission) from (through) a flexible structure by active means which is referred to as Active Structural Acoustic Control (ASAC). The ASAC scheme is an effective solution for low frequency applications which takes advantage of vibrating structural elements as secondary noise sources to cancel the sound fields generated by a primary noise source (A. Sampath, et al., “Active Control of Multiple Tones Transmitted in an Enclosure”, Journal of the Acoustical Society of America, Vol. 106, No. 1, Pages 211-225, July 1999; M. Al-Bassyiouni, et al., “Zero Spillover Control of Enclosed Sound Fields”, SPIE's Annual International Symposium of Smart Structures and Materials, Newport Beach, Calif., March 4-8, Vol. 4362, Paper No. 4326-7, 2001; and, M. Al-Bassyiouni, et al., “Experimental Studies of Zero Spillover Scheme for Active Structural Acoustic Control Systems”, Proceedings of the 12th International Conference on Adaptive Structures and Technologies (ICAST), University of Maryland, College Park, Md., Oct. 15-17, 2001). It appears that ASAC schemes require much less dimensionality than Active Noise Control (ANC) schemes in order to realize widely distributed spatial noise reduction. However, active research is still being pursued to address issues such as sensors, actuators, and control architecture.
Fiber-optic sensors have the advantages of being lightweight, having high sensitivity, and simplicity in multiplexing. Since the original demonstrations showed that optical fibers could be used as acoustic sensors (Bucaro J. A., et al., “Fiber Optic Hydrophone”, Journal of Acoustical Society of America, 62, Pages 1302-1304, 1977; and, Cole, J. H., et al., “Fiber Optic Detection of Sound”, Journal of Acoustic Society of America, 62, Pages 1136-1138, 1977), substantial research has been ongoing in this field. Much of this effort has been directed towards the development of hydrophones for ultrasonic detection which does not suit the needs for an ASAC system.
Since Bragg grating sensors were shown to be multiplexed by using Wavelength Division Multiplexing (WDM) techniques, Baldwin, et al., (“Bragg Grating Based Fabry-Perot Sensor System for Acoustic Measurements”, Proceedings of the SPIE 1999 Symposium on Smart Structures and Materials, Newport Beach, Calif., Mar. 1-5, 1999), developed a Bragg grating based Fabry-Perot sensor system for use in ASAC schemes. However, the sensor bandwidth was found to be limited and in addition, the sensor was found to have low sensitivity due to the high Young's modules of silica resulting in “acoustically induced strains” which also limit the application of these types of sensors.
Hence, low finesse Fabry-Perot sensors have become attractive choices for high performance sensing in this area. For example, a Fabry-Perot optical sensing device for measuring a physical parameter, described in U.S. Pat. No. 5,392,117, comprises a Fabry-Perot interferometer through which a multiple frequency light signal having predetermined spectral characteristics is passed. The system further includes an optical focusing device for focusing at least a portion of the light signal outgoing from the Fabry-Perot interferometer and a Fizeau interferometer through which the focused light signal is passed.
The Fabry-Perot interferometer includes two semi-reflecting mirrors substantially parallel to one another and spaced apart so as to define a Fabry-Perot cavity having transmittance or reflectance properties which are effected by a physical parameter such as pressure, temperature, refractive index of a liquid, etc., and which causes the spectral properties of the light signal to vary in response to the changes in physical parameters.
The Fabry-Perot interferometer is provided with at least one optical fiber for transmitting the light signal into the Fabry-Perot cavity and for collecting the portion of the light signal being transmitted outwardly. The Fizeau interferometer includes an optical wedge forming a wedge-profile Fizeau cavity from which exits a spatially-spread light signal indicative of the transmittance or reflectance properties of the Fabry-Perot interferometer.
The physical parameters can be determined by means of the spatially-spread light signal. It is however clear that in the sensing device of '117 Patent, the Fabry-Perot interferometer is a read-out interferometer rather than a sensor interferometer which is a Fizeau interferometer.
There are two types of modulation schemes used in fiber optical sensing systems to recover a signal, one being an intensity modulation scheme and the other being a phase modulation scheme. Intensity modulated sensors offer simplicity of design and ease of implementation, however they suffer from problems of limited sensitivity, low dynamic range and drift, due to uncertainty fluctuation. While phase modulated sensors are based on detection of the acoustically induced optical phase shift by using an interference technique. They have high sensitivity and don't suffer from optical source and receiver drift problems, however their non-linear input/output characteristics require careful demodulation design.
It is therefore clear that a sensor system for acoustic pressure measurements in a wide frequency range (50 Hz to 5.5-7.5 kHz) using thoroughly designed digital phase demodulation techniques is still needed in the field. In addition, as part of the ASAC scheme, other sensor configurations, which may be used for measuring sound pressure gradients, velocity and acceleration, are still of essential interest.
Velocity sensors have numerous advantages, some of which are as follows: (1) sensitivity to spherical waves than pressure microphone; (2) may be used along with the pressure microphones to measure the sound energy density, and (3) may be used along with pressure microphones to develop a unidirectional microphone which would favor waves incident from only one direction and discriminate from waves incident from other directions.
The concept of a typical velocity microphone is known in the prior art. However, complexity and bulkiness of known velocity microphones makes them difficult to use effectively in ASAC systems. A conventional arrangement of a velocity microphone consists of a corrugated metallic ribbon suspended between magnetic pole pieces N and S and freely acceptable to acoustic pressures on both sides (L. E. Kinsler, et al., “Fundamentals of Acoustics”, Second Edition, John Wiley & Sons, Inc., New York, 1962).
The ribbon acts as a short light cylinder that may be easily displaced in one direction under a force generated by air pressure. A velocity sensor was proposed (J. W. Parkins, “Active Minimization of Energy Density in a Three-Dimensional Enclosure”, Ph.D. Dissertation, Pennsylvania State University, 1998) which consists of six pressure condensor microphones mounted on a sphere of radius of 2.0 inches.
A finite difference scheme was used to predict the air particle velocity from the pressure measurement. Although the size of the sensor was “small” compared to many commercially available velocity probes, it was shown that such a sensor could lead to errors if there is any mismatching between the different pressure microphones.
There is also the potential for interference, since the microphones are housed together in a small volume. This interference may significantly affect the sensor signal-to-noise ratio, especially at low sound pressure levels. It is thus clear that a velocity sensor free of disadvantages of prior art velocity sensors is still needed in the industry.
As part of ASAC systems as well as for Active Vibration Control (AVC), acceleration sensors of high sensitivity and low mass can also contribute to overall control of structural vibration and interior noise. The conventional accelerometer consists mainly of a uniform cantilever beam fixed to the accelerometer housing and which is attached to a structure, the parameters of which are to be detected and measured. As the accelerometer vibrates due to base excitation, the cantilever tip oscillates about the undeformed axis, and the deflection at any point along the undeformed axis of the beam is a function of the excitation acceleration. It would be desirable to apply principles of the fiber tip based Fabry-Perot sensor to measurements of such a deflection of the oscillating beam.
Summarizing the discussion of the prior art supra, it is readily understood to those skilled in the art that it is still a long-lasting need in the field of active structural acoustical control to provide a wide bandwidth (in the frequency range of 50 Hz to 7.5 KHz) fiber tip based Fabry-Perot sensor systems for acoustic measurements with an extensively designed digital phase demodulation technique, free of disadvantages of the prior art acoustical measurement systems, and which would be capable of serving as microphone, velocity sensor, and acceleration sensor.